This invention relates to incandescent lamps and to methods and apparatus for manufacturing the same. The invention is more particularly concerned with lamps of axial geometry, with the manufacture of filament units and lamp envelopes, and with the precise alignment of the filaments in the envelopes.
Lamps of microminiature size now being manufactured have an envelope comprising two pieces of metal tubing sealed into the ends of a central glass sleeve. The filament consists of a tungsten helix dragged into the envelope through the end pieces and positioned so as to be approximately centered with respect to the central glass sleeve and to have an end extending into each piece of metal tubing. The lamp is sealed by cold-weld pinch-offs through the pieces of metal tubing and the ends of filament helix extending therein. Such lamps are disclosed in the applicant's prior application Ser. No. 846,466, filed July 31, 1969, now U.S. Pat. No. 3,736,630, granted June 5, 1973. Such lamp construction and method of filament insertion have made possible the economical production of microminiature size lamps by eliminating costly hand labor on each lamp. However, because of variations in the very small end openings of the pieces of metal tubing after cutting and processing, neither the points where the ends of the helix contact the metal tubing nor the number of helix turns between the two pieces of tubing can be controlled precisely. This makes it necessary to select and group the lamps from even the same production run according to light output and electrical characteristics.
The applicant has previously disclosed apparatus and methods for making microminiature lamp envelopes of axial geometry in which the metal end pieces are inserted axially into the central glass sleeve from opposite ends and are then heat-sealed to the glass section. Such apparatus and methods are disclosed in the applicant's prior application, Ser. No. 760,852, filed Sept. 19, 1968, now U.S. Pat. No. 3,578,429, granted May 11, 1971. While these apparatus and methods perform admirably in the production of microminiature envelopes, it is difficult to obtain even heating of the joints for larger size envelopes and to maintain concentricity.